St John treated 15 patients, two with serious injuries, after the bus crashed on a "lethal" stretch of State Highway 94 between Mossburn and Te Anau this morning.
Emergency services were called to the crash at 8.10am, where the bus rolled on its side at the bottom of Gorge Hill near The Key.
St John spokesman Gerard Campbell said four patients were transported by helicopter to Dunedin Hospital, two with serious injuries and two with moderate injuries.
Another two patients were being transported from Fiordland Medical Centre in Te Anau by helicopter to Southland Hospital with moderate injuries.
The ambulance service earlier said 19 people were treated, but this had since been updated.
Police said the rest of the hurt passengers all received minor to moderate injuries and were transferred by bus to the Fiordland Medical Centre.
Police said all 17 passengers on the bus, including the driver and the tour guide, had been taken from the scene.
The road was earlier closed, but the westbound lane was now open and stop-go traffic management was in place.
The New Zealand Transport Agency said the road was clear and fully reopened at 2pm.
The Commercial Vehicle Safety Team was at the scene as well as the Serious Crash Unit, and police photography.
"It's people just not driving to the conditions - I could have put my hat on it that it was going to happen this morning."
Mr McLean said another rental car had slid off the road this morning about 1km from where the bus crashed.
Today was the third frosty day in a row and there was a dangerous combination of frost mixed with fog.
"Nine times out of 10 the accidents happen on a straight road when speed becomes a factor."
Mr McLean praised the rescue helicopter crews that arrived to treat the injured bus passengers, saying they had done an amazing job.
The incident showed why rescue helicopters needed to continue operating from Te Anau, after The National Ambulance Sector Office (Naso), put air rescue services out to tender following a review.
''This is definitely why we need the rescue helicopters based here.''
A photograph of the crash scene supplied to the Otago Daily Times showed a white tourist bus, which was understood to be heading towards Milford Sound, on its side.
There were also signs of a heavy frost, with grass on the side of the road white.
Fire and Emergency New Zealand southern communications acting shift manager Ian Littlejohn said three people had to be cut free from the bus.
NZTA earlier warned road users about black ice on the stretch of road.
A woman who runs a nearby holiday home said she had not seen the crash but conditions on the highway were icy, especially in shaded areas, and it was also foggy.
''That bloody gorge it's lethal,'' she said of the crash site.
Te Anau chief fire officer Graeme Moffat said Te Anau was blanketed in fog and a black ice warning was in place this morning.
''You generally get black ice around the edge of the fog ... and there's a few shaded spots in the gorge.''
It is understood the passengers were heading towards Milford Sound for a cruise.
- With NZME
